Precise filtration processes, such as reverse osmosis, nano-filtration, ultra-filtration and micro-filtration, are used within a wide range of applications, including seawater desalination, fruit juice production and industrial wastewater treatment, among others. Synthetic filtration membranes, commonly referred to as semipermeable membranes, are typically used in conjunction with precise filtration processes. Semipermeable membranes provide selective mass transport, generally allowing the molecules of the solvent, but not the solute, to pass through their thickness. Semipermeable membranes are generally a thin layer of polymer, such as a layer of cast film. Synthetic filtration membranes may be formed from a number of polymers, including cellulose acetate, polyamide, polysulfone, polyvinylidene fluoride polymers and the like.
The process conditions encountered during filtration can be fairly rigorous. For example, elevated pressures of up to about 1200 psi may be employed to separate salt from seawater using reverse osmosis. Unfortunately, semipermeable membranes are typically relatively fragile. Laminate filter constructions incorporating a supporting substrate along with the semipermeable membrane are known to improve the durability of filtration media. Exemplary supporting substrates include a variety of porous materials, including sintered materials and nonwoven fabrics.
Laminate filter constructions were traditionally formed by casting the semipermeable membrane directly onto the supporting substrate. More recently, composite membranes, often referred to as Thin Film Composites (“TFC”) have been developed. Composite membranes include a porous polymer layer, such as a porous polymer film, in addition to the semipermeable membrane. The porous polymer layer is typically located between the semipermeable membrane and the supporting substrate. The intermediate porous polymer layer allows much thinner semipermeable layers to be used, yielding higher flux rates.
During filtration, the ingressing liquid stream is typically transported through the semipermeable or composite membrane initially, exiting through the supporting substrate. Consequently, the supporting substrate must provide strength properties while having a minimal effect on the semipermeable or composite membrane's transport properties, e.g. permeability or flux.
Suitable supporting substrates, e.g., suitable nonwoven fabrics, exhibit a number of other advantageous properties, as well. For example, the supporting substrate should exhibit acceptable adhesion to the intermediate porous polymer layer or semipermeable membrane, to avoid delamination during filtration. Suitable adhesion may be achieved by allowing the intermediate porous polymer layer or semipermeable membrane to penetrate down into the surface of the supporting substrate. However, the penetration of the supporting substrate by the intermediate porous polymer layer or semipermeable membrane represents a delicate balance. Inadequate penetration yields unacceptable adhesion within the filter media. Over penetration of the supporting substrate, e.g., penetration by the intermediate porous polymer layer or semipermeable membrane to the surface opposing the cast surface, results in uneven filtration properties (e.g. reduced flux) and/or damage of the semipermeable membrane due to the partial excessive pressurization during filtration.
In addition to the properties described above, supporting substrates should further advantageously provide a suitably smooth surface on which to apply the intermediate porous polymer layer or semipermeable membrane. Surface imperfections, particularly surface projections, create pinholes within the intermediate porous polymer layer and/or the semipermeable membrane, detrimentally affecting filter performance.
Supporting substrates made from nonwoven wet-laid fibers have been found to provide an advantageously smooth surface and acceptable affinity to semipermeable membranes. Exemplary wet-laid nonwoven webs intended for use as semi-permeable membrane supports are described in U.S. Pat. No. 5,851,355 to Goettmann, hereby incorporated by reference. Supporting substrates formed from nonwoven wet-laid fiber webs have been commercially available under the product name MEMBACK® nonwovens, from BBA.
Composite support constructions may be used to improve the economics of semipermeable filtration media, especially filtration supports incorporating wet-laid nonwoven webs. For example, U.S. Pat. Nos. 4,728,394 and 4,795,559 describe membrane supports that include a carded fiber layer bonded to a wet-laid web. However, although porous membranes incorporating carded webs provide a number of beneficial properties, such laminates can suffer from an unacceptable level of pinholes within the semipermeable or composite membrane. Carded webs are further a relatively expensive substrate.
Consequently, a need remains for composite supports incorporating wet-laid fiber webs that provide improved surface properties. There further remains a need for composite supports incorporating wet-laid webs that can be produced more economically.